1. Field
The present invention relates to an optical sheet for a backlight unit (BLU) of a liquid crystal display apparatus and a method of manufacturing the same, and a liquid crystal display apparatus using the optical sheet. More particularly the present invention relates to an optical sheet having a single sheet structure for an edge-lighting backlight unit using a printless light guide plate in a liquid crystal display apparatus, and to a method of manufacturing the same, and to a liquid crystal display, apparatus using the optical sheet.
2. Description of the Related Art
The light guide plates that can be used in the edge-lighting backlight units of LCDs usually fall into the two categories of a prism light guide plate, which uses only light refraction, and a non-printing light guide plate, which uses the scattering and condensation of light. FIG. 1 schematically illustrates a backlight unit using the prism light guide plate. As illustrated in FIG. 1, the backlight unit comprises a prism light guide plate 11 having a prism pattern 10 formed on the bottom surface thereof, and a reverse prism sheet 13 disposed on the prism light guide plate 11 and having a prism pattern 12 formed on the bottom surface thereof. Although a single sheet on the prism light guide plate may be achieved using the prism light guide plate illustrated in FIG. 1, the viewing angle is too narrow and it is difficult to manufacture the prism light guide plate, and it is not used more but rather less. FIG. 2 schematically illustrates a backlight unit using the printless light guide plate. As illustrated in FIG. 1, this unit comprises a printless light guide plate 20, a diffusion sheet 21 bonded to the upper surface of the printless light guide plate 20, a prism sheet 22 bonded to the upper surface of the diffusion sheet 21 and having a prism pattern 23 formed on the upper surface of the prism sheet, and a protective sheet 24 bonded to the upper surface of the prism sheet 22. Also, the backlight unit using the printless light guide plate may be configured without the protective sheet 24. In the case of the printless light guide plate using the scattering and condensation of light, which is easier to manufacture than the prism light guide plate, there is provided an optical sheet comprising a combination of two or three sheets of a diffusion sheet/a prism sheet or a diffusion sheet/a prism sheet/a protective sheet as illustrated in FIG. 2. FIG. 3 illustrates the direction of travel of light caused by the backlight unit using two sheets, an upper and a lower prism sheet. As illustrated in FIG. 3, the diffusion sheet 31 functions to screen the pattern of the light guide plate 30 which scatters light and also enables the obliquely directed light emitted from the light guide plate 30 to travel to some extent in the perpendicular direction. The prism sheet 32 functions' to condense the light, which has traveled to some extent in the perpendicular direction by means of the diffusion sheet 31, so that the light is sent in the perpendicular direction. As illustrated in FIG. 3, the backlight unit for notebook PCs condenses light in the completely perpendicular direction using two sheets comprising the lower vertical prism sheet 32 and the upper horizontal prism sheet 33. However, the backlight unit for LCD-TVs uses a single horizontal prism sheet because the use of two prism sheets may increase luminance but decrease the viewing angle from both sides. The prism sheet is problematic because prism-shaped ridges are exposed to the surface and may thus easily generate scratches upon handling or after assembly. To solve this problem or improve the viewing angle, a protective sheet may be disposed on the prism sheet.
However, the case where the printless light guide plate is adopted in the backlight unit for LCD-TVs is problematic because a plurality of optical sheets is used, undesirably making it impossible to achieve slimness, decreasing assembly productivity and increasing the cost of the backlight unit.
Thus there is required a single sheet for a backlight unit using the printless light guide plate in LCD-TVs in order to achieve slimness, increase assembly productivity and reduce the manufacturing cost.